Ultrananocrystalline Diamond Films with Optimized Dielectric Properties for Advanced RF MEMS Capacitive Switches

ABSTRACT

An efficient deposition process is provided for fabricating reliable RF MEMS capacitive switches with multilayer ultrananocrystalline (UNCD) films for more rapid recovery, charging and discharging that is effective for more than a billion cycles of operation. Significantly, the deposition process is compatible for integration with CMOS electronics and thereby can provide monolithically integrated RF MEMS capacitive switches for use with CMOS electronic devices, such as for insertion into phase array antennas for radars and other RF communication systems.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The United States Government has rights in this invention pursuant toContract No. DE-ACO2-06CH11357 between the U.S. Department of Energy(DOE) and UChicago Argonne, LLC representing Argonne National Laboratoryand pursuant to Contract No, MIPR06-W238 between the Defense AdvancedResearch Projects Agency (DARPA) and UChicago Argonne, LLC representingArgonne National Laboratory.

BACKGROUND OF THE INVENTION

This invention relates to capacitive switches and more particularly, toradio frequency (RF) microelectromechanical systems (MEMS) capacitiveswitches and a fabrication process therefore.

RF MEMS capacitive switches have many useful applications for militaryand commercial RF and microwave applications, RF MEMS capacitive switchcomprises a movable metal membrane suspended above a lower electrode andan interposing dielectric layer. An air gap of several microns typicallyseparates the upper membrane from the dielectric layer. The lowerelectrode comprises a RF signal path, while the upper electrodecomprises a RF and DC ground. In the switch “off state”, the air gapbetween the membrane and lower electrode is sufficient that the uppermembrane has an insignificant parasitic capacitance relative to theoperating frequency of the switch. When a voltage is applied across theupper and lower electrodes, the electrostatic force pulls the membranedown into contact with the dielectric layer (“on state”). Without asignificant air gap, the upper metal membrane, insulator layer, andlower metal electrode form an MIM (metal-insulator-metal) capacitor.This capacitor is designed to achieve sufficient capacitive conductancesuch that it can capacitively couple, or even short, the RF signal pathof the lower electrode to the grounded upper metal membrane. When theapplied voltage is released, the restoring force of the membrane metalspring is sufficient to return the membrane to its “off state”.

Electronic switching devices comprise radio frequencymicroelectromechanical systems (RF MEMS) have many potential benefitsover conventional semiconductor devices for controlling and routingmicrowave and millimeter-wave signals. RF MEMS switches possess very lowinsertion loss, miniscule power consumption, and ultrahigh linearity.These characteristics make MEMS switches ideal candidates forincorporation into passive circuits, such as phase shifters or tunablefilters, for implementation in communications and radar systems at 1 GHzand above.

Despite the excellent RF performance of these devices, their acceptancein industry has been limited by a lack of reliability. In awell-engineered MEMS switch, dielectric charging is the main limitationto lifetime, as opposed to mechanical effects. When the switch actuates,a relatively high voltage (30-50 volts) is applied across a relativelythin switch insulator. The resulting electric field induces chargetunneling into the insulator, where they trapped. As these charges buildup, they shift the pull-in and release voltages of the switch. If enoughcharges become trapped, the operating voltages will shift sufficientlysuch that the switch will either remain stuck down, or not actuate whendesired. In either case, the switch fails to operate properly.

Furthermore, while the RF performance of these devices can be exemplary,reliability issues have limited their deployment into fielded systems,in the case of capacitive MEMS switches, shortcomings relating todielectric charging have been difficult to mitigate. There are manysolutions for lessening the impact of dielectric charging, includinghermetic packaging, minimizing the electric field across the dielectric,and tailoring the polarity and waveform of bias control signals tominimize charging. These solutions have provided significantimprovements in reliability, but have not proven enough to overcome the“stigma” associated with dielectric charging.

Commercially available RF switches use silicon dioxide (SiO₂) or siliconnitride (Si₃N₄) as a dielectric layer material in a capacitive switch.Charges become trapped in the layer and charge builds over time. As thecharge builds, the operation of the device degrades until it fails. Infact, it fails very slowly. Studies have shown that the charge anddischarge time constants for these materials are on the order of 10s ofseconds to 100s of seconds. After failure, a device may take days torecover because charges trapped in the dielectric layer take so long torecombine at the metal electrodes of the capacitor. The amount of chargeaccumulated is exponentially related to the applied electric field. Thehigher the operating voltages, the longer the switches are left in the“on state”. Furthermore, the higher the operating temperature, generallythe faster the switch will fail.

More specifically, conventional prior art capacitive switches with oxideor nitride dielectric layers are chosen or designed such that thecharges accumulate as slowly as possible. Conventional prior switchesslowly degrade until the point of failure. Switches with oxide ornitride dielectrics also possess inherently long discharging timeconstants. Charging and discharging time constants are approximatelyequal. Therefore, once failure has occurred, conventional prior artdevices are not available for proper operation for a very long timeperiod, rendering the device essentially useless for a majority ofapplications and uses.

The primary failure mode of conventional prior art RF MEMS capacitiveswitches is accumulation of electrically charged particles within theinsulator layer made of silicon oxide or silicon nitride materials ofthe switch, in which charges tunnel into and become trapped within thedielectric. The conventional prior art RF MEMS capacitive switch onlyrecovers from this failure after a sufficiently long period of time(hours to days) during which the trapped charges can diffuse or migrateback to the metal electrodes. However, for practical purposes,conventional prior art RF MEMS capacitive switches often fail since themembrane remains stacked to the dielectric layer covering the bottomelectrode.

Several techniques have been developed to mitigate the effects ofdielectric charging on switch reliability, such as minimizing theoperating conditions that lead to dielectric charging, for example,time, voltage, temperature; however, the designer does not often havecontrol over these parameters. Alternatively, design modifications canbe made to the switch to enable more reliable operation. One alternativeis to minimize the amount of dielectric material within the switch toform a mechanical support of the membrane layer. The dielectricinsulating material is patterned into “posts” which support themembrane, but minimize the amount of contact between the dielectric andmembrane. Instead of a metal-insulator-metal capacitive switch, it ismore properly described as a metal-air-metal switch. This modificationtrades capacitance ratio (ratio of on capacitance to off-capacitance)for improved reliability.

An alternative method of reducing dielectric charging is to engineer thechemical makeup of the dielectric such that it is conductive, or leaky.Given sufficient conduction within the dielectric, the trapped chargeswill have more opportunity to recombine in the device current, andthereby be eliminated. However, depending on the physics of the particlecharging and discharging, the quiescent current may not always be theproper mechanism for causing the induced charges to dissipate, in whichthe quiescent current provides no substantial advantage.

There have been attempts to manipulate the bulk conductivity of thedielectric film to bleed off charges and improve reliability.Unfortunately, these techniques have not proven repeatable or sufficientenough to be generally adopted.

Use of diamond or diamond-like carbon (DLC) films as a dielectric for RFMEMS capacitive switches has been suggested. Nanocyrstalline diamondfilm can be grown using a bias enhanced nucleation (BEN) process at ahigh temperature (700° C.). Tetrahedral amorphous carbon (ta-C) film canbe fabricated for use as a dielectric layer in RF MEMS capacitive switchwith improved dielectric properties. However, the diamond-like carbon(DLC) films exhibit high as-grown stress and need to be annealed above600° C. to release relieve internal stress. Since both BEN grown diamondand DEC films mentioned above involve high temperature processing eitherduring growth or after deposition, they are not compatible forintegration with complementary metal-oxide-semiconductors (CMOS)electronics and, therefore, their usefulness is limited, since theycannot be used to fabricate monolithically integrated RF MEMS switcheswith CMOS devices, which is the ultimate device architecture of interestto manufacturers, who want these integrated devices for insertion intophase array antennas for radars and other RF communication systems. Thedielectric properties of ultrananocrystalline diamond (UNCD) films grownat high temperatures have been studied before. However, no reports havebeen published to date on tuning the dielectric properties of UNCD grownat low temperatures compatible with the CMOS thermal budget, whichprovides the total amount of energy transferred to a wafer at a givenelevated temperation operation (such as ˜100° C.) and their use in RFMEMS switches.

Furthermore, the typical thickness of the dielectric layer is a few 100sof nm and it is challenging to deposit such a thin diamond film withoutany pin-holes.

It is, therefore, desirable to provide an improved RF MEMS capacitiveswitch and fabrication process therefore, which overcomes most, if notall of the preceding problems.

BRIEF SUMMARY OF THE INVENTION

An improved RF MEMS capacitive switch and fabrication process thereforeis provided for use in controlling, routing, and tuning RF wirelesscircuits and systems, as well as for other uses and applications.Advantageously, the improved RF MEMS capacitive switch is reliable,effective and efficient. Significantly, the fabrication process iscompatible thr integration with complementary metal-oxide-semiconductors(CMOS) electronics and thereby can provide monolithically integrated RFMEMS switches for use with CMOS electronic devices, such as forinsertion into phase array antennas for radars and other RFcommunication systems.

RF MEMS capacitive switches incorporating ultrananocrystalline diamond(UNCD) dielectric films integrated, with CMOS can provide orders ofmagnitude better switching performance and higher durability in harsherenvironments. RF MEMS capacitive switch offer significantly greaterperformance for military and commercial communications applications.

This invention provides multiple layers ultrananocrystalline diamond(UNCD) as a dielectric for reliable operation of radio frequencymicroelectromechanical systems (RF MEMS) capacitive switches. Moreparticularly, the insulating layers on top of the electrode within theRF MEMS capacitive switch are dielectric films with electrical leakycharacteristics that has properties optimized to enable a reliable,long-life, such as over 1-200 billion cycles of operation of the RF MEMScapacitive switch. The material used as a dielectric with controlledleakage in the RF MEMS capacitive switch is specially fabricatedultrananocrystalline diamond (UNCD) thin film.

In order to fabricate the RF MEMS capacitive switch UNCD thin films asdielectric layers with controlled leakage to eliminate membrane stiction(static friction) when closing the RF MEMS capacitive switch, a uniquemicrowave plasma chemical vapor deposition (CVD) deposition process isprovided. This inventive process carefully optimizes process parametersto deposit UNCD thin films at low temperature (400-450° C.) with uniquedielectric properties to enable a reliable, long-life, such as over1-200 billion cycles of operation of the RF MEMS capacitive switch. Morespecifically, this inventive process involves engineering the UNCDdielectric film to have sufficiently short discharging time constantsthat no matter how much charges are accumulated in the dielectric layer,the RF MEMS capacitive switch will recover quickly enough so as to beavailable for proper operation within a relatively short time span(microseconds to tens or hundreds of microseconds).

The inventive fabrication process preferably includes a three stepprocess for depositing UNCD films with optimized dielectric propertiesfor use in an RF MEMS capacitive switch. The UNCD films preferablycomprise multiple dielectric layers that cover the bottom electrode inthe RF MEMS capacitive switch. Use of UNCD for the dielectric layersprovide reliable, long life operation of the RF MEMS capacitive switchby eliminating the problem of dielectric charging which is theaccumulation of charge within the dielectric layer.

The new three-step growth process for producing UNCD layers,specifically provides control of the amount of hydrogen (H₂)incorporated at the grain boundaries of the UNCD layers andsimultaneously provides tailoring of the grain size of the individuallayers grown with different amounts of H₂ added to the argon(Ar)/methane (CH₄) base gas mixture. The controlled incorporation ofhydrogen (H) atoms at the grain boundaries provides controlledsatisfaction of dangling bonds, which result in a dielectric layer withtailored dielectric constant and charge conduction through the grainboundaries. This produces fast charging of the layer and also quickdischarge (leak) of the charges first accumulated in the dielectriclayers during operation of the switch. The fast charging/dischargingperformance of the multilayered UNCD films provides a new paradigm in RFMEMS switch operation with fast recovery of the switch (≦80 μsec) asopposed to 10-100 sec, for conventional dielectric materials, such assilicon dioxide (SiO₂) or silicon nitride (Si₃N₄), thereby practicallyeliminating failure due to dielectric charging and is 5 to 6 timesfaster than the charge/discharge characteristics of conventionalswitches using SiO₂ and Si₃N₄ materials as dielectric layers.

The three layer approach not only helps to form ultra-thin films(200-300 nm) without pin-holes but also helps to control the internalstresses in the UNCD films. Additionally, chemically inert andhydrophobic nature of UNCD eliminates stiction and tribo-chargingproblems comprising triboelectric generation or a type of charging withthe contacting metal membrane.

The fabrication technique can include a three-step process to depositUNCD films with optimized dielectric properties.

1. The first step comprises reducing the hydrogen content by volume to1% 2% using argon (Ar)/methane (CH₄)/hydrogen (H₂) gas chemistry in themicrowave chemical vapor deposition (CVD) plasma during initialdeposition period to allow rapid nucleation of UNCD and forming acontinuous thin film.

2. In the second step, the H₂ percentage is increased to 4%-5% in theAr/CH₄/H₂ gas chemistry in the CVD plasma to increase incorporation ofH₂ at the grain boundary so as to help provide for the unique dielectricproperties of the film.

3. in third step, the H₂ percentage is reduced back to the 1% 2% used instep 1 and chemical vapor deposition (CVD) is continued for a short timeto complete the CVD of the thin films providing the multiple dielectriclayers. This allows dense renucleation of nanodiamond and fill-in of anygaps between grains, and forms a uniform continuous layer. This 3-layerstep approach not only helps to from ultra-thin films (200-300 nm)without pin-holes, but also helps to reduce overall stress and providesgood dielectric properties.

The unique multilayer approach developed has been demonstrated for itsintegration with complementary metal-oxide-semiconductor (CMOS) and canbe useful in reducing overall compressive stress in UNCD film, which canimprove the overall yield of the devices, which is subjected tostringent processing conditions during micro-fabrication of RF MEMScapacitive switches.

Preferably, the improved process for use in fabricating a RF MEMScapacitive switch, comprises: providing a bottom electrode; and layeringultrananocrystalline diamond (UNCD) on the bottom electrode to provide amultilayered UNCD dielectric film having electrical leakycharacteristics on the bottom electrode for fast dielectric charging anddischarging and rapid recovery of the RF MEMS capacitive switch.Advantageously, the layering comprises depositing layers of UNCD bymicrowave plasma chemical vapor deposition (CVD) with microwave CVDplasma. The preferred process includes: forming a layer of substantiallycontinuous dielectric film with rapid nucleation of UNCD by decreasinghydrogen (H₂) content of the microwave CVD plasma; thereafter increasinghydrogen (H₂) concentration in a grain boundary of an intermediate layerof the dielectric film by increasing hydrogen (H₂) content of themicrowave CVD plasma to form a high resistivity layer of film with ahydrogen-enriched grain boundary; and subsequently decreasing hydrogen(H₂) content of the microwave CVD plasma for dense renucleation of theUNCD for filing in gaps or pinholes between grains and forming asubstantially uniform continuous layer. The microwave CVD plasma cancomprises argon (A_(R)), methane (CH₄) and hydrogen (H₂).

The fabrication process can include reducing stress in the dielectricfilm, placing a membrane above the dielectric film and positioning anupper electrode above the membrane. Advantageously, the RF MEMScapacitive switch is fabricated and designed to operate for at least 45million cycles when driven by a CMOS electronic device.

A more detailed explanation of the invention is provided in thefollowing detailed descriptions and appended claims taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a RF MEMS capacitive switch in accordancewith principles of the present invention.

FIG. 2 is a cross-sectional view of the RF MEMS capacitive switch,

FIG. 3 is a diagram and chart illustrating discharge data for the RFMEMS capacitive switch.

FIG. 4 is a diagram and chart illustrating open state insertion lossmeasurements of the RF MEMS capacitive switch.

FIG. 5 is a diagram and chart illustrating a switch life-time of the RFMEMS capacitive switch.

FIG. 6 is a diagram and chart illustrating the new layeredultrananocrystalline diamond (UNCD) synthesis approach.

FIG. 7 is a diagram and chart illustrating the hydrogen (H)concentration in the film as a function of the H concentration in theplasma.

FIG. 8 is an image of 2% hydrogen (H₂) in the plasma.

FIG. 9 is an image of 6% hydrogen (H₂) in the plasma.

FIG. 10 is a diagram and chart illustrating the dissipation factor andtest frequency.

FIG. 11 is a perspective view of a microwave plasma chemical vapordeposition (MPCVD) system.

FIG. 12 is a perspective view of 400° C. ultrananocrystalline diamond(UNCD) films with ±5% thickness variation over 150 mm diameter silicon(Si) wafer.

FIG. 13 is a perspective view of 400° C. UNCD film deposited on 150 mmdiameter sapphire-CMOS wafer with ±5% thickness variation.

FIG. 11 is a top plan view of 400° C. UNCD film deposited on 150 mmdiameter sapphire-CMOS wafer with ±5% thickness variation.

FIG. 15 is an enlarged top plan view of a portion of 400° C. UNCD filmdeposited on 150 mm diameter sapphire-CMOS wafer with ±5% thicknessvariation.

FIG. 16 is a perspective view of a wireless diamond MEMS/CMOS device,such as a mobile cellular phone.

FIG. 17 is a perspective view of a high frequency, low loss, RF MEMScapacitive switch with UNCD dielectric layers monolithically integratedwith CMOS compatible devices on a single chip.

FIG. 18 is a diagram and chart illustrating effect of hydrogen (H)termination on the adhesion between different surface and tip materialsincluding a tungsten carbon (W₂C) tip and a diamond tip before and afterH termination.

FIG. 19 is a perspective view of a diamond tip before hydrogen (H)termination,

FIG. 20 is a perspective view of a diamond tip after hydrogen (H)termination.

FIG. 21 is a diagram illustrating demonstration of integration of UNCDwith CMOS.

FIG. 22 is a focused ion beam (FIB) cross-section image of UNCD/CMOS.

FIGS. 23-24 are diagrams and charts illustrating post UNCDcharacterization of P-type metal oxide semiconductor (PMOS) and N-typemetal oxide semiconductor (NMOS) devices before and after UNCDdeposition.

FIG. 25 is a diagram of monolithically integrated UNCD RF MEMScapacitive switches on sapphire substrates.

FIG. 26 is a diagram of a cross-section view of a portion of amonolithically integrated UNCD RF MEMS capacitive switch on a sapphiresubstrate.

FIG. 27 is an image of CMOS on a sapphire wafer coated with UNCD anddemonstrating the integration of UNCD with CMOS on sapphire.

FIG. 28 is an enlarged image of CMOS on a sapphire wafer coated withUNCD and with silicon nitride (Si₃N₄) and platinum (Pt).

FIG. 29 is an image of UNCD with a film thickness of about 70 nm.

FIGS. 30-31 are further diagrams and charts illustrating post UNCDcharacterization of P-type metal oxide semiconductor (PMOS) and N-typemetal oxide semiconductor (NMOS) devices before and after UNCDdeposition.

FIG. 32 is a diagram and chart illustrating number of cycles as afunction of applied voltage and showing that UNCD based RF MEMS switchesexhibit superior lifetime as compared to silicon nitride (Si₃N₄) basedswitches.

FIG. 33 is a diagram illustrating monolithically integrated RF MEMScapacitive switches/silicon on sapphire (SOS)-CMOS devices fabricatedand tested using layered. UNCD dielectric films.

FIG. 34 is an image of a SOS CMOS wafer.

FIG. 35 is a top plan view of a RF MEMS capacitive switch.

FIG. 36 is a diagram and chart of a bipolar pulsed actuation curveillustrating capacitance as a function of actuation voltage of a UNCDbased monolithically integrated fully functional RF MEMS capacitiveswitches driven by CMOS.

FIG. 37 is an image of prior nanocrystalline diamond film grown at hightemperature of 800° C. and which is not compatible with CMOS.

FIG. 38 is an image of a fabricated metal-insulator-metal (MIM)capacitor with diamond as well as chromium (chrome) (Cr)/platinum (Pt)and titanium (Ti)/gold (Au).

DETAILED DESCRIPTION OF THE INVENTION

The following is a detailed description and explanation of the preferredembodiments of the invention and best modes for practicing theinvention.

Referring to the drawings, a radio frequency (RF) microelectromechanical(MEMS) capacitive switch 100 (FIGS. 1 and 2) is moveable for a switchingtime from an off position in an off state to an on position in an onstate. The RF MEMS capacitive switch can have: a bottom electrode 102comprising metal that provides a RF signal path; a top electrode 104comprising an RF ground and a direct current (DC) ground; a moveablemetallic membrane 106, and multiple layered (multi-layered) dielectricfilms 108 with electrical leaky characteristics and providing multipledielectric insulating layers on the bottom electrode. The RF MEMScapacitive switch can also have undercut access holes 110. The RF MEMScapacitive switch can have a substrate 112, such as a silicon onsapphire (SOS) substrate 114, positioned below the bottom electrode andcan have at least one post 116 that extends between the membrane and thesubstrate. Advantageously, the RF MEMS capacitive switch can comprise areliable long-life switch that is operable for more than 100 billioncycles.

In the preferred embodiment, the moveable metallic membrane comprisesmolybdenum (MO) and has high conductivity, low resistance and ismechanically robust. The membrane can be less than 0.4 μm thick and canbe spaced from the bottom electrode by an air gap 118 of about 2 micronsto about 10 microns resulting in a substantially insignificantcapacitance relative to an operating frequency of the switch. In use,the membrane contacts the dielectric film when a voltage of about 30volts to about 50 volts is applied across the top and bottom electrodesin the on state. Desirably, the membrane cooperates with the insulatinglayer and the bottom electrode to form a metal-insulator-metal (MIM)capacitor 120 (FIG. 35) in the on state.

Advantageously, the RF MEMS capacitive switch can be monolithicallyintegrated with one or more complementary metal-oxide-semiconductors(CMOS) electronic devices 122 (FIGS. 13, 14, 16, 27, 28, and 33).Co-integration with CMOS makes RF MEMS capacitive switch more userfriendly and can also vastly improve life testing and lifetime withintelligent control electronics. Monolithically integrated RF MEMSswitches can be used with CMOS electronic devices, such as for insertioninto phase array antennas for radars and other RF communication systems.RF MEMS capacitive switches incorporating ultrananocrystalline diamond(UNCD) dielectric films integrated with CMOS can provide orders ofmagnitude better switching performance and higher durability in harsherenvironments, RF MEMS capacitive switches offer significantly greaterperformance for military and commercial communications applications.

In the preferred embodiment, the dielectric films compriseultrananocrystalline diamond (UNCD) substantially without pinholes,gaps, voids. The dielectric films preferably have a discharging andrecovery time substantially less than the switching time forsubstantially discharging an accumulated charge within 50 microseconds.In the illustrative embodiment, the dielectric films provides dielectriclayers which provides leaky dielectric layers with a charging timeconstant of about 100 microseconds.

Use of UNCD provides controlled leakage and allows reliable, long lifeoperation of the RF MEMS capacitive switch by eliminating the problem ofdielectric charging which is the accumulation of charge within thedielectric layer. The UNCD dielectric material of the RF MEMS capacitiveswitch has charge and discharge time constants that are on the order ofmicroseconds to 10s of microseconds instead of 100s of seconds forconventional RF MEMS capacitive switches with an insulating layer ofsilicon dioxide (SiO₂) or silicon nitride (Si₃N₄). UNCD films haveunique charging properties with a very fast time constant which allowlonger lifetimes that possible with traditional dielectrics.

As shown in FIG. 6, the multi-layer dielectric films comprisingultrananocrystalline diamond (UNCD) preferably include: (1) a bottomUNCD layer 124 which provides a lower ultra thin film that covers asubstantial area of the upwardly facing portion of the bottom electrode;(2) an intermediate UNCD layer 126 on the bottom UNCD layer thatprovides a middle ultra thin film and a high resistivity film, and (3)an upper UNCD layer 128 that provide atop ultra thin film on theintermediate UNCD layer. The intermediate UNCD layer preferably has ahydrogen enriched grain boundary 130 for enhanced charge conduction.

In the illustrative embodiment, the bottom electrode comprises tungsten(W) or a stack comprising chromium (chrome) (Cr), tungsten and chromiumso as to accommodate high temperature UNCD deposition.

Desirably, the inventive RF MEMS capacitive switch recovers quicklyenough for the switch to be available for operation after only a short(in microseconds) interruption followed by a rapid recovery beforenormal operation. Advantageously, UNCD has low stiction properties, ischemically inert, hydrophobic, and reduces problems related totribiological interaction of metal membrane with dielectric material.

The process for fabricating a radio frequency (RF)microelectromechanical switch (MEMS) capacitive switch, can comprise thesteps of: providing a substrate, placing a bottom electrode on thesubstrate, and depositing layers of ultrananocrystalline diamond (UNCD)thin films on the bottom electrode by microwave chemical vapordeposition (CVD) with a microwave CVD plasma gas comprising argon (Ar),methane (CH₄) and hydrogen (H₂) to form multilayered UNCD dielectricfilms. As shown in FIG. 6, the preferred process includes: depositing abottom UNCD layer on the bottom electron with H₂ flow for rapidnucleation of the UNCD and form a substantially continuous dielectricfilm; depositing an intermediate UNCD layer on the bottom UNCD layerwith a higher H₂ flow and forming a higher resistivity film with ahydrogen enriched grain boundary; and depositing an upper UNCD layer onthe intermediate UNCD layer with a lower H₂ flow than for theintermediate UNCD layer for dense nucleation of the UNCD to fill in thegaps or pinholes between grains and forming a substantially continuouslayer.

In the fabrication process, the dielectric layers can layers can bedeposited at a temperature ranging from 400-500° C. with 400 sccm Ar and1-2 sccm CH₄. The bottom layer can be deposited with 5-8 sccm H₂. Theintermediate layer can be deposited with 14-16 sccm H₂. The upper layercan be deposited with 5-8 sccm H₂. The H₂ concentration can be increased4-5% by volume in the microwave CVD plasma for the intermediate layerand decreased by 1-2% by volume in the microwave CVD plasma for theupper layer. The dielectric films can comprise ultra thin UNCDdielectric films ranging from 200-300 nm.

The first layer (bottom layer) with low hydrogen flow allows rapidnucleation for forming a continuous UNCD film. The second layer(intermediate layer) with higher hydrogen flow facilitates deposition ofhigh resistivity film and high permittivity. The third layer (upperlayer) at low hydrogen flow allows dense renucleation and fill-in of anygaps (pinholes). The layered UNCD approach provides good dielectricproperties but also help to reduce the over stress in the film stack andprovides pin-hole free, continuous UNCD films at 200-300 nm thickness.Preferably, all the UNCD layers are grown at or less than 450° C. withinthe CMOS thermal budget.

The fabrication process can include positioning a moveable metallicmembrane above the multilayered UNCD dielectric film and positioning atop electrode above the membrane. Desirably, the fabrication processproduces, forms and provides a long life RF MEMS capacitive switch withthe multilayered UNCD dielectric films, membrane, electrodes andsubstrate so that the RF MEMS capacitive switch is operable for morethan 100 billion cycles. The RF MEMS capacitive switch discharges andleaks charges accumulated in the dielectric film so that the RF MEMScapacitive switch recovers within at least 80 μsec. This process andstructure substantially prevents failure of the RF MEMS capacitiveswitch due to dielectric charging of the films and can substantiallyeliminate stiction and tribo-charging problems when the RF MEMScapacitive switch is closed and the membrane contacts the dielectricfilm.

The preferred process can comprise fabricating a RF MEMS capacitiveswitch in which the membrane comprises molybdenum (Mo), the bottomelectrode comprises tungsten (W) or a stack comprising chromium (chrome)(Cr), tungsten and chromium, and the bottom electrode is positioned on asubstrate comprising silicon on sapphire (SOS) or a silicon wafer.

Desirably, the process can include monolithically integrating the RFMEMS capacitive switch with a complementary metal-oxide-semiconductor(CMOS) electronic device.

FIG. 3 is a diagram and chart illustrating discharge data for the RFMEMS capacitive switch with the UNCD dielectric layers. FIG. 3 showsthat the discharge time is in the range of microseconds, which areorders of magnitude shorter than for conventional switches with asilicon dioxide (SiO₂) or silicon nitride (Si₃N₄) dielectric layer. TheRF MEMS capacitive switch with the UNCD dielectric layers workedsuccessfully for more than 1 billion cycles.

FIG. 4 is a diagram and chart illustrating open state insertion lossmeasurements of the RF MEMS capacitive switch with UNCD dielectriclayers. The performance was at 10 GHz and indicates an insertion loss of0.17 dB. This is extracted from S-pars with: C_(off) at −16 ff, C_(on)at −644 ff, and a C_(ration) of −41. The RF MEMS capacitive switch withthe UNCD dielectric layers provide low loss, harsh environment, longlifetime UNCD based switches.

FIG. 5 is a diagram and chart illustrating a switch life-time of the RFMEMS capacitive switch with the UNCD dielectric layers which workedsuccessfully for more than one billion cycles.

The development of UNCD films as a leaky dielectric is shown in FIGS.7-10. UNCD films were grown with 2% hydrogen (H₂) at 800° C. provide:higher volume fraction of grain boundaries, higher amount of bulk H₂,higher conductivity and higher dissipation. FIG. 7 is a diagram andchart illustrating the hydrogen (H) concentration in the film as afunction of H concentration in the plasma. FIG. 8 is an image of 2%hydrogen (H₂) in the plasma. FIG. 9 is an image of 6% hydrogen (H₂) inthe plasma. FIG. 10 is a diagram and chart illustrating the dissipationfactor as a function of test frequency.

FIG. 11 is a perspective view of a 915 MHz microwave plasma chemicalvapor deposition (MPCVD) system at the Center for Nanoscale Material(CNM) at Argonne National Laboratory which was used fabricate andproduce 400° C. ultrananocrystalline diamond (UNCD) films with ±5%thickness variation over 150 mm diameter silicon (Si) wafer as shown inFIG. 12 and 400° C. UNCD film deposited on 150 mm diameter sapphire-CMOSwafer with ±5% thickness variation, as shown in FIGS. 13-15.

FIG. 17 is a perspective view of a high frequency, low loss, RF MEMScapacitive switch with UNCD dielectric layers monolithically integratedwith CMOS compatible devices on a single chip. This provides UNCDMEMS/nanoelectromechanical systems (NEMS) in which UNCD deposition canoccur at 400° C. and can be useful for piezoelectically actuated diamondbased switches, resonators and fillers, as well as high frequency (GHz)lower insertion loss devices for single chip mounted CMOS driven RF MEMScapacitive switches. They can be used with leverage economics of largevolume in diamond MEMS/CMOS devices, such as cell phones and otherwireless devices. Furthermore, they can be implemented in highperformance, high reliability integrated diamond MEMS/CMOS wirelesssensors and actuator devices in warfare systems and defenseapplications.

FIG. 18 is a diagram and chart illustrating effect of hydrogen (H)termination on the adhesion between different surface and tip materialsincluding a tungsten carbon (W₂C) tip and a diamond tip before and afterH termination. Record low adhesion energies approaching van der Waal'slimits: ≈30 mJ/m² for methyl terminated hydrocarbons FIG. 19 is aperspective view of a diamond tip before hydrogen (H) termination. FIG.20 is a perspective view of a diamond tip after hydrogen (H)termination.

FIG. 21 is a diagram illustrating demonstration of integration of UNCDwith CMOS. A tungsten (W) layer can be used to facilitate UNCD seedingand growth. Plasma enhanced chemical vapor deposition (PECVD) silicondioxide (SiO₂) can be used for isolation to avoid shortening betweenpads and the tungsten layer. Aluminum (Al) can be used as a hard maskfor reactive ion etching (RIE) of the UNCD as well as for PECVD. BothN-type metal oxide semiconductor (NMOS) and P-type metal oxidesemiconductor (PMOS) devices were tested before and after UNCDdeposition. FIG. 22 is a focused ion beam (FIB) cross-section image ofthe UNCD/CMOS.

FIGS. 23-24 are diagrams and charts illustrating post UNCDcharacterization of P-type metal oxide semiconductor (PMOS) and N-typemetal oxide semiconductor (NMOS) devices before and after UNCDdeposition including an extra PECVD layer and the pad reopening process.The solid lines illustrate a fresh ship, while the dashed linesillustrate a chip with UNCD. In FIG. 23, the threshold voltage (Vth) wasincreased by 0.08 volts (V) and the transconductance (g_(m)) decreasedby 1.5%. In FIG. 24, the threshold voltage (Vth) was increased by 0.03volts (V) and the transconductance (g_(in)) decreased by 1.5%. The postUNCD CMOS was sufficient to drive on-chip integrates MEMS devices.

FIG. 25 is a diagram of monolithically integrated UNCD RF MEMScapacitive switches on sapphire substrates. FIG. 26 is a diagram of across-section view of a portion of a monolithically integrated UNCD RFMEMS capacitive switch on an insulating sapphire substrate. Among theadvantages of the monolithically integrated UNCD RF MEMS capacitiveswitches on insulating sapphire substrates are: (a) ultra CMOSeliminates parasitic capacitance due to the insulating substrate; (b)extremely fast switching due to lack of capacitance; (c) no non-linearvoltage dependent capacitance; (d) no cross-talk or leakage through thesubstrate; and (d) enable excellent speed, linearity and isolation.

FIG. 27 is an image of CMOS on a sapphire wafer coated with UNCD anddemonstrating the integration of UNCD with CMOS on sapphire. FIG. 28 isan enlarged image of CMOS on a sapphire wafer coated with UNCD and withsilicon nitride (Si₃N₄) and platinum (Pt). FIG. 29 is a focused ion beam(FIB) image of UNCD with a film thickness of about 70 nm.

FIGS. 30-31 are further diagrams and charts illustrating post UNCDcharacterization of P-type metal oxide semiconductor (PMOS) and N-typemetal oxide semiconductor (NMOS) devices before and after UNCDdeposition. They show a significant shift in the threshold voltage afterUNCD deposition and processing. The CMOS on sapphire sample without anyprocessing was measured and taken as a control sample. Thereafter, a 10nm thick tungsten layer was deposited. A 100 nm thick UNCD thin film wasdeposited followed by reactive ion etching (RIE) of the UNCD. Laseretching of the tungsten layer was done to open the pads.

FIG. 32 is a diagram and chart illustrating number of cycles as afunction of applied voltage and showing that UNCD based RF MEMS switchesexhibit superior lifetime as compared to silicon nitride (Si₃N₄) basedRF MEMS switches.

FIG. 33 is a diagram illustrating monolithically integrated RF MEMScapacitive switches/silicon on sapphire (SOS)-CMOS devices fabricatedand tested using layered UNCD dielectric films. In FIG. 33 at differentplaces, there were four CMOS drive circuits, two RF MEMS capacitiveswitches, four MEMS test structures, two MEMS 1-bit phase shifters andone CMOS test structure. FIG. 34 is an image of a silicon on sapphire(SOS) CMOS wafer. The die size was 10.1 mm×10.1 mm. The die includesCMOS driver circuits, CMOS test structures. MEMS switches and 1-bitphase shifter, as well as MEMS test structures. The 150 mm wafercontained 130 dies. FIG. 35 is a top plan view of a RF MEMS capacitiveswitch.

FIG. 36 is a diagram and chart of a bipolar pulsed actuation curveillustrating capacitance as a function of actuation voltage of a UNCDbased monolithically integrated fully functional RF MEMS capacitiveswitches driven by CMOS. The CMOS wafer was diced. The circuitry wastested for functionality and successfully worked for 85 million cycles.

FIG. 37 is an image of prior nanocrystalline diamond film grown at hightemperature of 800° C. and which is not compatible with CMOS, FIG. 38 isan image of a fabricated metal-insulator-metal (MIM) capacitor withdiamond as well as chromium (chrome) (Cr)/platinum (Pt) and titanium(Ti)/gold (Au).

The deposition parameters of the diamond film are shown in Table 1.

TABLE 1 DEPOSITION PARAMETERS OF THE DIAMOND FILM Process Parameter BENGrowth Gas flow rate (CH₄:H₂) [sccm] 10:200 2:200 Plasma Power [W] 300300 Pressure [torr] 15 15 Substrate Bias [V] −250 −125

Diamond growth a high temperatures, such as 800° C., are not compatiblewith CMOS.

The insertion loss of coplanar waveguide (CPW) of three materialconfigurations for diamond, silicon dioxide (SiO₂) are silicon (Si) areshown in Table 2. This was done on MIM capacitors.

TABLE 2 INSERTION LOSS OF CPW ON THREE MATERIAL COFIGURATIONS Frequency(i) (ii) (iii) (GHz) (dB/cm) dB/cm) (dB/cm) 10 1.2 1.4 1.0 30 1.7 1.81.4 65 2.6 2.3 2.1

The ultrananocrystalline diamond (UNCD) thin films incorporated into RFMEMS capacitive switches can be deposited using microwave plasmachemical vapor deposition (MPCVD) and hot filament chemical vapordeposition (HFCVD) techniques. In both cases, UNCD films can bedeposited at moderate substrate temperatures of 500° C.-680° C. TheMPCVD grown UNCD films were produced in a 915 MHz MPCVD system installedin the Center for Nanoscale Materials at Argonne National Laboratoryusing a DiamoTek 1800 series brand 915 MHz, 10 KW MPCVD system fromLambda Technologies, Inc.

A special process can involve using seeding pretreatment followed byUNCD deposition using Ar/CH₄/H₂ gas chemistry to achieve diamond filmswith specific dielectric properties. The UNCD nucleation and growthprocess can result in a unique film microstructure with equiaxed 3 nm-5nm grains and 0.4 nm wide grain boundaries.

A TEM micrograph has been made of ˜230 nm thick UNCD film deposited onthe 150 mm diameter quartz wafer used for incorporation into RF MEMScapacitive switches.

Characterization of the UNCD film using Raman spectroscopy and near edgex-ray absorption fine structure spectroscopy (NEXAFS) revealed that theUNCD film is high quality with very high percentage (98%) of sp3 bondedcarbon. The UNCD synthesis parameters can be optimized to achieve filmson quartz substrates with excellent thickness uniformity such as with avariation in thickness of ±5% from center to edge across 150 mm diametersubstrates.

The improved process can comprise substituting new materials for thesubstrate and the lower electrode to accommodate the higher depositiontemperature of diamond (>400° C.) relative to silicon dioxide (<100°C.). The substrate material can be changed from borosilicate glass(Pyrex) to quartz and sapphire. Both substrates permit higher processtemperatures, and sapphire substrates enable the RF MEMS capacitiveswitch process to be compatible with silicon-on sapphire (SOS)electronics for CMOS-MEMS co-integration. Additionally, the standardchrome/gold (Au)/chrome metal stack for the lower electrode was replacedwith a stack comprising chrome (chromium) (Cr)/tungsten (W)/chrome. Thisnew electrode stack was designed to withstand higher depositiontemperatures without significant metal migration or diffusion. Tocompensate for the fact that tungsten has a resistivity more than doublethat of gold, the tungsten film thickness was doubled to beapproximately 0.5 μM thick. This helps maintain the switch insertionloss at a low level.

The 3-layer UNCD step approach can improve the dielectric properties aswell as control internal stress in the dielectric films. The hydrogenbonding present at the grain boundary can help discharge chargesaccumulated in the dielectric layer rapidly, resulting in a fastrecovering of the switch (≦80 μsec) and thereby practically eliminatingfailure due to charging. This process provides an improved RF MEMScapacitive switch with 5-6 times order of magnitude improvement in thecharging-discharging characteristics of the RF MEMS capacitive switchwith UNCD dielectric layers over conventional prior art RF MEMScapacitive switch with silicon dioxide (SiO₂) and silicon nitride(Si₃N₄) materials as dielectric layers.

RF MEMS capacitive switches can be fabricated with an insulating layerwhich comprises a dielectric film that is positioned on top of theelectrode. Significantly, the dielectric film has electrical leakycharacteristics and has properties optimized to enable a reliable,long-life (≧100 billion cycles) operation of the RF MEMS capacitiveswitch. Preferably, the material used as a leaky dielectric comprisesultrananocrystalline diamond (UNCD) developed in thin film form.Desirably, the UNCD dielectric film has sufficiently short dischargingtime constants that no matter how much charges are accumulated in thedielectric layer, the RF MEMS capacitive switch will recover quicklyenough so as to be available for proper operation within a relativelyshort time span (microseconds to tens or hundreds of microseconds).

Generally, insulating materials for conventional prior art RF MEMScapacitive switches are engineered to avoid dielectric charging and failvery slowly. However, once failure occurs, the recovery time for theswitch to be operational again is very long, essentially resulting inpermanent failure of the switch and the system in which it is inserted,as a critical component. With this invention, the RF MEMS capacitiveswitch with a UNCD dielectric film can experience what colloquially isnamed and sometime erroneously described as a “failure”, but in realityis a short operational interruption (microseconds long), followed by arapid recovery (in microseconds) to normal operation. In this instance,the so called “failure” of the switch, due to dielectric charging, ismoot and the RF MEMS capacitive switch will operate very effectivelydespite the charging effect.

The improved radio frequency (RF) microelectromechanical systems (MEMS)capacitive switch has a quick switching time and can be moved from anoff state to an on state. The RF MEMS capacitive switch can have: alower electrode, an upper electrode, a membrane that is positionedbetween the electrodes, and a leaky dielectric layer that is position onthe lower electrode. Significantly, the leaky dielectric layer has adischarging time that is substantially less than the switching time. Inuse, the membrane is spaced from the electrodes in the off state and themembrane contacts the dielectric layer when a voltage is applied acrossthe electrodes in the on state.

The dielectric layers can have electrical leaky characteristics and cancomprise an insulating layer. Desirably, the dielectric layers arechemically inert and hydrophobic to substantially prevent tribiologicalinteraction of the membrane with the dielectric layers. Preferably, thedielectric layers comprise ultrananocrystalline diamond (UNCD). Thedielectric layers can comprises film with a grain size of about 3 nm toabout 5 nm.

Advantageously, the RF MEMS capacitive switch can recover quickly in aperiod of time ranging from 1 microsecond to 100 microseconds from anaccumulation of electrically charged particles on the insulating layer.The RF MEMS capacitive switch is designed and arranged to prevent itfrom failing due to rapid discharging of the dielectric layer.Desirably, the RF MEMS capacitive switch can operate with a dischargingtime less than 50-100 microseconds and is operable for over 100 billioncycles.

The inventive RF MEMS capacitive switch provides improved reliability.Advantageously, the switch dielectric is designed to have a dischargingtime constant which is short relative to the required switching time ofthe device. Since a well-designed micromechanical switches typicallychange state in less than 50 microseconds, it is desirable to have anyaccumulated charge to dissipate within 50 microseconds.

Significantly, traditional failure of the insulator in prior artswitches due to dielectric charging is irrelevant, as the inventive RFMEMS capacitive switch with a UNCD dielectric film recovers quicklyenough to be ready for the next operating cycle. Desirably, the mainfailure mode for capacitive MEMS switches is circumvented by thisinvention and the RF MEMS capacitive switch will have a good reliabilityset by the mechanical characteristics of the UNCD dielectric film.

Further improvements can be achieved if the charging time constant ismade as long as possible. This causes the switch to accumulate charge ata slower rate than one with a short time constant. Irrespective of thecharging time constant, if the discharging time constant is made shortenough, then an induced charge will quickly dissipate and the devicewill recover to normal operation very quickly.

In order to provide a long life and reliable RF MEMS capacitive switchwith short discharging time constants, the RF MEMS capacitive switchuses a ultrananocrystalline diamond (UNCD) as the switch dielectricinstead of conventional silicon oxide or nitride dielectrics. Due to theultra-small grain sizes (3-5 nm) characteristic of UNCD, themean-free-path to conductive grain boundaries is very short, allowingany trapped charges to recombine very quickly. Measurements made on thecharging time constants of UNCD demonstrate a charging time constant ofapproximately 100 microseconds.

While measurements of the specific discharging time constant can bedifficult to quantify due to a variety of mitigating circumstances, inthe inventive RF MEMS capacitive switch, full discharging has beendemonstrated to be less than 50 microseconds for switch on-times as longas 0.5 seconds.

The paradigm, pattern or example of operation with the inventive RF MEMScapacitive switch is fundamentally different from that of a traditionalcapacitive switch. For RF MEMS capacitive switch with a UNCD dielectricfilm, the discharge time is in the range of microseconds, which isorders of magnitude shorter than for traditional prior art switches witha silicon oxide or silicon nitride dielectric.

Significantly, the RF MEMS capacitive switch has ultrananocrystallinediamond (UNCD) as the switch dielectric. The impact on electromechanicalperformance is minimal. However, these devices exhibit uniquelydifferent charging characteristics, with charging and discharging timeconstants 5-6 orders of magnitude quicker than conventional materials.The RF MEMS capacitive switches with a UNCD dielectric layer (film) canprovide devices which have no adverse effects of dielectric charging andcan be operated near-continuously in the actuated state withoutsignificant degradation in reliability.

Characterization of the electromechanical switch properties typicallyincludes measuring its dynamic operating curve. These properties weremeasured by sweeping bias voltage and measuring the capacitance of thedevice (C-V curves). These switches exhibit actuation voltages in therange of 30-45 volts with an on-capacitance ranging from 650 fF to 800fF. The on-capacitance is primarily determined by the surface roughnessof the lower electrode and the cleanliness of the sacrificial releaseprocess. The measured switch off-capacitance ranged from 90 fF to 105ff, which includes 52 fF of transmission line capacitance. This meansthat the MEMS plate and fringing capacitance is approximately 38 fF-53fF.

The DC I-V characteristics of the switch were also measured as part ofthis characterization. When actuated, 5-25 nA of leakage current flowedthrough the switch, depending heavily on the operating voltage. To date,there has not been any perceived correlation between UNCD leakagecurrent and the charging properties of these switches.

The RF performance of this switch is typical of most MEMS capacitiveswitches. When the shunt switch is in the off-state, the insertion lossis very low, on the order of 0.25 dB at 20 GHz. This is slightly higherthan the usual insertion loss of 0.15 dB, and is attributed to the lessconductive metal for the lower electrode. When the switch is actuated,isolation is set by the on-capacitance of the device. With 700 fF ofon-capacitance, the isolation at 20 GHz is very close to the theoreticalvalue associated with a shunt 700 fF capacitor (7.7 dB). This switchoperates more like a switched capacitor (45 fF-725 fF) than a highisolation switch at frequencies below 20 GHz.

Traditional switch dielectrics such as SiO₂ and typically have hulkcharging and discharging time constants of 10 to 100+ microseconds. Asthe switches operate, charges build up within their dielectric, and theyexperience a very gradual change in pull-in and release voltages untilthe ultimate failure of the switch. With bulk charging, this failure ischaracterized by the release voltage dropping to zero and the devicebecoming stuck down. After failure, the switch requires a sufficientlylong period of time to recover (in which the charges recombine) beforeit is able to release.

After careful measurements, it was determined that switches with UNCDdielectrics have time constants that are 5-6 orders of magnitude quickerthan those of conventional materials, which are on the order of 100 μS.With these very short time constants, the switch fails very quickly,less than a millisecond after actuation. As is characteristic of bulkcharging, the device becomes stuck down. However, after the switch biasis removed, the switch requires a very short time for the charges todissipate and the switch to release. The switch recovery time is not thetypical 5-10 μS switching speed, but is dependent on the amount ofcharging which occurred during and after failure. Therefore, the timerequired for recovery and release is dependent on the switch on-time.

The release time of the switch as a function of the transmitted RFpower, is the switch on-time. The switch recovery time, t^(OFF), cantake many hundreds of microseconds depending on the switch on-time,t^(ON). In the most extreme cases, switches required milliseconds torecover. The switch recovery time can be plotted as a function ofon-time, but there can be a saturation effect to the induced charge, sothe recovery time does not continue to grow significantly with extendedswitch on-time.

The reason for this uniquely different operation is that the chargingand discharging time constants of the dielectric layer are very short.As the RF MEMS capacitive switch is actuated and charges, bulk chargingaids in the actuation of the switch and there is no perceived differencein actuation time. However, the release of the switch is delayed untilthe accumulated charges have had sufficient time to recombine and/ordissipate. This has the effect of making the switch release timedependant on the switch on-time and subsequent dissipation ofaccumulated charges.

To better understand the charging phenomenon, pulsed s-parametermeasurements can be measured to investigate the charging characteristicsof the RF MEMS capacitive switch with a UNCD dielectric layer.

A voltage waveform can be used to study dielectric charging of theswitches. First, a control voltage of V^(ON)≧33 V can used to pull inthe switch and to charge the dielectric for different t^(ON) times.Next, the control voltage is reduced to V^(OFF)=0 and, at 10 μs afterthe control voltage is reduced from V^(ON) to V^(OFF), the switchcapacitance is sampled to see whether or not the switch is released.Usually, the mechanical release process takes less than 10 μs tocomplete. Therefore, if the switch is released, its capacitance shoulddecrease significantly. Once the switch is confirmed to have beenreleased, V^(OFF) is incremented by 0.1 V for the next charging cycle,after t^(OFF)≧20 ms to ensure most of the charge is discharged and theswitch returns to its pristine state at the beginning of the nextcharging cycle. This way, the cycles are repeated until the switch flitsto release after 10 μs and the V^(OFF) then is deemed the releasevoltage. The release voltage decreases monotonically from 13 V to 0 whenthe charging time is increased from 10 to 500 μs.

Because the magnitude of the release voltage decreases after charging,charging appears to be in the bulk instead of the surface of UNCD. Byassuming the charge distributed in the bulk of UNCD has the same effectas a sheet charge in the middle of the film, the shift in releasevoltage can be fitted to the following formula:

ΔV=(dQ ₀/2∈₀∈_(R))[1−exp(−t ^(ON)/τ_(C))]

where d is the UNCD thickness, Q₀ is the steady-state charge density, 0is the vacuum permittivity, ∈_(R)=5.2 is the diamond film dielectricconstant, and τ_(C) is the charging time constant. The measuredrelease-voltage shift can be best fitted with τ_(C)=95 μs andQ₀=2×10¹²/cm².

Table 3 lists the charge densities and time constants. These results arein contrast to charging in SiO₂ or SiN_(x), which involves timeconstants on the order of 10 sec and charge density in the order of 10¹¹q/cm², under comparable fields of 10⁶ V/cm.

TABLE 3 Charge Density and Time Constant of UNCD Switches Di- Di-electric electric Charge Thick- Con- Stress Density Time ness stantVoltage Q0 (10¹²/ Constant Wafer d (μm) ε_(R) Switch V^(on) (V) cm²)τ_(c) (ms) 1 0.33 5.2 06C-0110 +35 2.0 0.095

The measurements imply that the UNCD possesses very short charging anddischarging time constants. Charge carriers are concentrated at thegrain boundaries, and since the grains are nano-sized, they do not havefar to travel to diffuse into or out of the dielectric. In essence, theswitch pulls down and immediately charges to failure. However, when theapplied voltage is removed, the charges leave the dielectric veryquickly, depending on how long the switch was in the down position. Itis easy to envision conditions under which the switch will recover fromcharging within a designated switching time interval (e.g. 50 μS). Withthis mode of operation, the switch will always recover fully fromcharging before the next switch operation ensues.

RF MEMS capacitive switches with UNCD dielectric layers can providethree order of magnitude quicker recovery times than conventional priorart RF MEMS capacitive switches. Switches left “on” for 100 secondsrecovered back to their original condition in less than 50 μS. Thisimplies that if switches are cycled off once out of every 100 seconds,they will be fully recovered from any effects of charging and are readyto be reused anew. This means that they only have to be turned off0.00005% of the timeline to operate without charging failure. Continuousswitch operation is now reasonably achievable for RF MEMS capacitiveswitches because of this invention.

The measured results for these switches demonstrate a new paradigm inswitch operation, one that potentially alleviates the scourge ofdielectric charging. Presently, film stresses are high and delaminationis a common occurrence in conventional prior art RF MEMS capacitiveswitches. Furthermore, pin-hole effects have been experienced inconventional prior art RF MEMS capacitive switches and steps must betaken to reduce their occurrence.

RF MEMS capacitive switches have been fabricated with this new approachon 150 mm diameter complementary metal-oxide-semiconductor(CMOS)-on-sapphire wafers (FIGS. 13-15) to achieve monolithicallyintegrated UNCD-based RF MEMS capacitive switches/CMOS. The performancehas been successfully tested on the inventive RF MEMS capacitive switchdriven by on-chip integrated CMOS.

FIGS. 13-15 illustrate images and photographs of 150 mm diameterCMOS-sapphire wafer with monolithically integrated UNCD RF-MEMS switchas shown in FIG. 1.

FIG. 36 shows the capacitance measurements taken on one of the RF MEMScapacitive switch as the RF MEMS capacitive switch closes and releaseswith the application of voltage. The operation of the switch isindicated by arrows as shown in the FIG. 36. As the voltages reaches toa minimum threshold pull down voltage, the molybdenum (Mo) membrane ispulled down to make physical contact with the UNCD dielectric layersforming an MIM (metal-insulator-metal) capacitor; this is called the “onstate”. This capacitor is designed to achieve sufficient capacitiveconductance such that it can capacitively couple, or even short, the RFsignal path of the lower electrode to the grounded upper metal membrane.When the applied voltage is released, the restoring force of themembrane metal spring is sufficient to return the membrane to its “offstate”.

FIG. 36 illustrates capacitance measurement for a RF MEMS capacitiveswitch driven by a complementary metal-oxide-semiconductor (CMOS) devicein the wafer shown in FIGS. 13-15. The RF MEMS capacitive switch withUNCD dielectric lasted for about 87 millions cycles of on-offoperations. This is the first demonstration of CMOS-driven RF MEMScapacitive switch based on diamond. The upper molybdenum (Mo) membrane,which forms the MIM capacitance also play an important role in thereliability of the RF MEMS capacitive switch since often timesconventional prior art RF MEMS capacitive switch switches fail due tothe mechanical failure of this membrane due to constant buckling action.

The deposition process helps solve the problem of films stress andachieving device yield better than 90% on a full 150 min diameter wafer,which is needed for commercialization.

This invention will be used in phased array high frequency radars, cellphones, etc.

The advantages of the inventive RF MEMS capacitive switches with UNCDlayers over conventional technologies include the ability to work inharsh environment and high frequency switching with very shortcharging-discharging time with an improvement over 5-6 order ofmagnitude over conventional prior art RF MEMS capacitive switches usingsilicon dioxide (SiO₂) or silicon nitride (Si₃N₄) dielectric layers.

The incorporation of ultrananocrystalline diamond films as an insulatingdielectric for RF MEMS switches has been demonstrated in this invention.The charging and discharging time constants for these films are on theorder of 100 μS and 5-6 orders of magnitude faster than those ofconventional insulating films. This enables switches to recover from theadverse effects of charging quickly enough that the impact can be madenegligible. For the first time, this offers the possibility of operatingcapacitive MEMS switches that are almost continuously “on” without anadverse impact on switch reliability.

RF MEMS capacitive switches with ultra-short recovery times areprovided. Tests of RF MEMS capacitive switches with a UNCD dielectriclayer have shown order of magnitudes shorter discharging times withrespect to RF MEMS switches using conventional silicon oxide or siliconnitride dielectric layers. RF MEMS capacitive switches with UNCDdielectric layers have been tested up to 12 billion cycles and showexcellent performance.

This invention circumvents the traditional “dielectric charging” failuremode common to capacitive RF MEMS switches. This enables ultra-low loss,low power consumption, and especially linear operation of MEMS devicesto be used in a manner that is repeatable and reliable.

RF MEMS capacitive switches with a UNCD dielectric layer (film) providevariable capacitors that can be used to control and route microwave andmillimeter-wave signals. Uses and applications for the inventive RF MEMScapacitive switches can include phase shifters for electronicallyscanned antenna arrays and tunable filters for spectrum control andanti-jamming, RF MEMS capacitive switches can be used in military andcommercial radar and communications systems.

RF MEMS capacitive switches with a UNCD dielectric layer can also beused in numerous applications which depend on reliable, immediatelyavailable performance: phase shifter for phase array antennas, cellphone communications, industrial automation, PC peripherals, automatictest equipment, medical devices, instruments and military/aerospaceequipment. RF MEMS capacitive switches with a UNCD dielectric layeroffer promise of cost effective, high performance devices over a widerange of applications, RF MEMS capacitive switches with a UNCDdielectric layer can operate in harsh environments with a much longeroperating life.

Among the many advantages of inventive RF MEMS capacitive switches witha UNCD dielectric layer (film) and process for producing the same are:

-   -   1. A superior fabrication and deposition process for RF MEMS        capacitive switches.    -   2. Compatible for integration with complementary        metal-oxide-semiconductors (CMOS) electronics.    -   3. Superb monolithically integrated RF MEMS capacitive switches        for use with CMOS electronic devices.    -   4. Excellent use with phase array antennas for radars and other        RF communication systems.    -   5. Orders of magnitude better switching performance.    -   6. Higher durability in harsh environments.    -   7. Greater performance for military and commercial        communications applications    -   8. Better Reliability,    -   9. Longer Life.    -   10. Shorter time constants.    -   11. Superior switch capabilities.    -   12. Outstanding performance.    -   13. User friendly.    -   14. Economical.    -   15. Efficient.    -   16. Effective.

Although embodiments of the invention have been shown and described, itis to be understood that various modifications, substitutions, andrearrangements of process steps, parts, components, and materials, aswell as other uses of the RF MEMS capacitive switch and process, can bemade by those skilled in the art without departing from the novel spiritand scope of this invention.

1. A process for use in fabricating a radio frequency (RF)microelectromechanical systems (MEMS) capacitive switch, comprising thesteps of: providing a bottom electrode; and layeringultrananocrystalline diamond (UNCD) on the bottom electrode to provide amultilayered UNCD dielectric film having electrical leakycharacteristics on the bottom electrode for fast dielectric charging anddischarging and rapid recovery of the RF MEMS capacitive switch.
 2. Aprocess for use in fabricating a RF MEMS capacitive switch in accordancewith claim 1 wherein the layering comprises depositing layers of UNCD bymicrowave plasma chemical vapor deposition (CVD) with microwave CVDplasma.
 3. A process for use in fabricating a RF MEMS capacitive switchin accordance with claim 2 including forming a layer of substantiallycontinuous dielectric film with rapid nucleation of UNCD by decreasinghydrogen (H₂) content of the microwave CVD plasma.
 4. A process for usein fabricating a RF MEMS capacitive switch in accordance with claim 2including increasing hydrogen (H₂) content of the microwave CVD plasmato form a high resistivity layer of film with a hydrogen-enriched grainboundary.
 5. A process for use in fabricating a RF MEMS capacitiveswitch in accordance with claim 4 including decreasing hydrogen (H₂)content of the microwave CVD plasma for dense renucleation of the UNCDfor filing in gaps or pinholes between grains and forming asubstantially uniform continuous layer.
 6. A process for use infabricating a RF MEMS capacitive switch in accordance with claim 2wherein said microwave CVD plasma comprises argon (A_(R)), methane (CH₄)and hydrogen (H₂).
 7. A process for use in fabricating a RF MEMScapacitive switch in accordance with claim 1 including reducing stressin the film.
 8. A process for use in fabricating a RF MEMS capacitiveswitch in accordance with claim 1 including increasing hydrogen (H₂)concentration in a grain boundary of an intermediate layer of thedielectric film.
 9. A process for use in fabricating a RF MEMScapacitive switch in accordance with claim 1 including: placing amembrane above the dielectric film; positioning an upper electrode abovethe membrane; and positioning the RF MEMS capacitive to operate for atleast 45 million cycles when driven by a complementarymetal-oxide-semiconductor (CMOS) electronic device.
 10. A process forfabricating a radio frequency (RF) microelectromechanical switch (MEMS)capacitive switch, comprising the steps of: providing a substrate;placing a bottom electrode on the substrate; depositing layers ofultrananocrystalline diamond (UNCD) thin film on the bottom electrode bymicrowave chemical vapor deposition (CVD) with a microwave CVD plasmagas comprising argon (Ar), methane (CH₄) and hydrogen (H₂) to formmultilayered UNCD dielectric films including depositing a bottom UNCDlayer on the bottom electron with H₂ flow for rapid nucleation of theUNCD and forming a substantially continuous dielectric film; depositingan intermediate UNCD layer on the bottom UNCD layer with a higher H₂flow and forming a higher resistivity film with a hydrogen enrichedgrain boundary; depositing an upper UNCD layer on the intermediate UNCDlayer with a lower H₂ flow than for the intermediate UNCD layer fordense nucleation of the UNCD to fill in the gaps or pinholes betweengrains and forming a substantially continuous layer; positioning amoveable metallic membrane above the multilayered UNCD dielectric film;positioning a top electrode above the membrane; and forming a long lifeRF MEMS capacitive switch with the multilayered UNCD dielectric film,membrane, electrodes and substrate so that the RF MEMS capacitive switchis operable for more than 100 billion cycles.
 11. A process forfabricating a RF MEMS capacitive switch in accordance with claim 10wherein: the layers are deposited with 400 sccm Ar and 1-2 sccm CH₄; thebottom layer is deposited with 5-8 sccm H₂; the intermediate layer isdeposited with 14-16 sccm H₂; and the upper layer is deposited with 5-8sccm H₂.
 12. A process for fabricating a RF MEMS capacitive switch inaccordance with claim 10 wherein the layers are deposited at atemperature ranging from 400-500° C.
 13. A process for fabricating a RFMEMS capacitive switch in accordance with claim 10 where in the H₂concentration is increased 4-5% by volume in the microwave CVD plasmafor the intermediate layer and decreased by 1-2% by volume in themicrowave CVD plasma for the upper layer.
 14. A process for fabricatinga RF MEMS capacitive switch in accordance with claim 10 wherein: thedielectric film comprises an ultra thin UNCD dielectric film rangingfrom 200-300 nm; discharging and leaking charges accumulated in thedielectric film and recovering the RF MEMS capacitive switch within atleast 80 μsec; and substantially preventing failure of the RF MEMScapacitive switch due to dielectric charging of the film.
 15. A processfor fabricating a RF MEMS capacitive switch in accordance with claim 10including substantially eliminating stiction and tribo-charging problemswhen the RF MEMS capacitive switch is closed and the membrane contactsthe dielectric film.
 16. A process for fabricating a RF MEMS capacitiveswitch in accordance with claim 10 wherein: the membrane comprisesmolybdenum (Mo); the bottom electrode comprises tungsten (W) or a stackcomprising chromium (chrome) (Cr), tungsten and chromium; the bottomelectrode is positioned on a substrate selected from the groupconsisting of silicon on sapphire (SOS) and a silicon wafer; and the RFMEMS capacitive switch is monolithically integrated with a complementarymetal-oxide-semiconductor (CMOS) electronic device.
 17. A radiofrequency (RF) microelectromechanical (MEMS) switch, comprising: a RFMEMS capacitive switch moveable for a switching time from an offposition in an off state to an on position in an on state, comprising; abottom electrode comprising metal providing a RF signal path and havingan upwardly facing portion; a top electrode comprising a RF ground and adirect current (DC) ground; a moveable metallic membrane less than 0.4μm thick, said membrane being spaced from said bottom electrode by anair gap ranging from about 2 microns to about 10 microns resulting in asubstantially insignificant capacitance relative to an operatingfrequency of the switch; a multi-layer dielectric film comprisingultrananocrystalline diamond (UNCD) including: a bottom UNCD layercovering a substantial area of the upwardly facing portion of the bottomelectrode; an intermediate UNCD layer on said bottom UNCD layer forproviding a high resistivity film, said intermediate UNCD layer having ahydrogen enriched grain boundary for enhanced charge conduction; anupper UNCD layer on the intermediate UNCD layer; and the dielectric filmcomprising a substantially continuous UNCD film substantially withoutpinholes; said membrane contacting said multi-layer dielectric film whena voltage ranging from about 30 volts to about 50 volts is appliedacross the top and bottom electrodes in the on state; and saidmulti-layer dielectric film discharging and leaking accumulated chargesand said RF MEMS capacitive switch recovering within 80 μsec.
 18. An RFMEMS switch in accordance with claim 17 wherein said RF MEMS switch isdesigned to operate for over 100 billion cycles.
 19. An RF MEMS switchin accordance with claim 17 wherein said multi-layer dielectriccomprises an ultra thin UNCD dielectric film ranging from 200 nm to 300nm.
 20. An RF MEMS switch in accordance with claim 17 wherein: themembrane comprises molybdenum (Mo); the bottom electrode selected fromthe group consisting of bottom electrode comprising tungsten (W) and astack comprising chromium (chrome) (Cr), tungsten and chromium; thebottom electrode is positioned on a substrate selected from the groupconsisting of silicon on sapphire (SOS) and a silicon wafer; and the RFMEMS capacitive switch is monolithically integrated with a complementarymetal-oxide-semiconductor (CMOS) electronic device.